10 Incredible Holes

The entrance to Krubera Cave is buried within the Western Caucasus mountain range in the Abkhazia region of Georgia. iStock/Thinkstock

Caves are among the world's most unpredictable holes; a small mountainside entrance could lead several feet or several thousand feet into the ground. This mysterious quality is what brought a team of Ukranian spelunkers to the bottom of Krubera Cave, situated deep within the Arabika Massif in the western Caucasus Mountains. This geological formation, located in the Abkhazia region of Georgia, is known for its deep caves; another of the world's deepest caverns, Sarma, begins its 5,062-foot (1,542-meter) descent into the Earth here. Krubera, however, travels 7,188 feet (2,191 meters) below ground, making it the deepest known cave on Earth, and the only cave known to descend more than 6,000 feet (1,829 meters) [source: Kasjan].

The cavers who descended into Krubera's extraordinary depths were a part of the Ukrainian Speleological Association's "Call of the Abyss" project, which was funded by the National Geographic Society. Between 1999 and 2010, the team made a number of expeditions into the cave's steep, narrow passageways, sometimes blasting their way through sections too small to negotiate. The most notable of these explorations occurred in October 2004 when the spelunkers first descended below the highly anticipated 2,000-meter (6,562-foot) mark. During this historic adventure, cavers remained underground for a difficult four weeks and, much like mountain climbers, established base camps at 2,300, 3,990, 4,600, and 5,380 feet (700 meters, 1,215 meters, 1,410 meters, and 1,640 meters) [source: Klimchouk]. A later expedition in 2007 extended the cave's explored depth to an incredible 7,188 feet (2,191 meters).